The invention is based on a priority application EP 02 360 016.6 which is hereby incorporated by reference.
The present invention relates to the field of optical filters.
As is known, a coded optical signal namely a signal carrying information such as an RZ or NRZ coded signal ((Non) Return to Zero) has intensity modulations. FIG. 1 shows the curve 1 representing the classical spectrum 1 of an NRZ coded signal. This spectrum is shown in terms of optical power P as a function of wavelength λ. This spectrum presents a profile with a carrier wavelength λc1 at a maximum optical power P1 and has two side-bands 10, 11 with respect to this carrier wavelength λc1.
The two side-bands 10, 11 of this NRZ spectrum contain redundant information. It is therefore attractive to filter out one of them in order to improve transmission performance. This technique, known as Vestigial Side-band (VSB) filtering, is used in DWDM (Dense Wavelength Division Multiplexing) transmission systems transmitting NRZ coded signals. It involves a narrow optical filter, narrower than the signal bandwidth, and which is offset with respect to the carrier wavelength.